The present invention relates to processes for the production of nanoporous polymer foams by depressurization and nanoporous foams obtainable thereby.
It is generally known that foamed plastics can be produced by extrusion of melts containing volatile propellants.
Thus in “Polymer Engineering and Science”, Vol. 38, No. 7, 1998, M. Lee et al. describe the extrusion of foamed polyethylene/polystyrene blends with supercritical carbon dioxide.
Particularly in the thermal insulation field, foams are used as insulating material. Since the mean free path of air is about 60 to 100 nanometers (depending on pressure and temperature), it can be concluded from this that in a polymer foam with air as the cell gas at an average cell size of less than or equal to 60 to 100 nanometers the contribution of the cell gas to the total thermal conduction of the foam is significantly reduced or even completely eliminated. Hence foams with as small-celled a structure as possible are especially desirable.
However it must be noted not only that the attainment of such a small cell size is important, but also that the foam density must be reduced as far as possible, in order not to lose the advantage gained via the cell gas through an increased contribution of the polymer matrix to the total thermal conduction. This means that a nanoporous foam must also have as low a density as possible in order to have an improved thermal insulating action compared to standard polymer foams.
In addition there is the problem that very small cell sizes can indeed often be present directly after the foaming, but then a maturation takes place with the formation of larger cells.
For example, in U.S. Pat. No. 5,955,511 and in EP-A 1 424 124 processes for the production of micro- and nanoporous polymer foams are described, in which in a first step a polymer is loaded with a propellant under pressure at low temperatures below the glass transition temperature of the polymer. After depressurization without foaming, this laden polymer is then foamed in a separate step by increasing the temperature.
In WO2008/087559, continuous extrusion processes for the production of nanoporous polymer foams are described, in which a polymer is admittedly exposed to the propellant at different temperatures under pressure, but the subsequent foaming process by depressurization is performed at very low temperatures far below the glass transition temperature of the pure polymer but above the glass transition temperature of the gas-laden system.
In US2009/0130420, a continuous extrusion process for the production of nanoporous polymer foams is described, in which a polymer melt is loaded with a propellant under pressure and is foamed by subsequent depressurization likewise in the region of the glass transition temperature of the gas-laden melt. Admittedly, high process pressures up to 1000 MPa are stated here for the loading, however the stated depressurization rate of 10 to 1000 MPa/s in combination with the low temperatures once again leads to a comparatively high foam density.
However, not only do the processes described have process technology disadvantages, but the product properties also reveal a need for further optimization.
The systems produced are often microporous or macroporous. Here “microporous” means that the pore sizes lie in the range from 1 to 1000 micrometers. The term “macroporous” designates dimensions greater than 1000 micrometers.
Hence the purpose of the present invention is to find processes for the production of nanoporous polymer foams with improved application technology properties, which enable deliberate adjustment of the cell size and the foam density with high and designated precision. Further, the processes should be simpler to perform than the known processes.